A new study records for the first time what happens to brain cells when patients go under.

Three patients undergoing treatment for epilepsy provided a unique opportunity for researchers led by Patrick Purdon at Massachusetts General Hospital and Harvard Medical School to study the effects of anesthesia on the brain. As part of the treatment protocol, electrodes implanted in their brains would help doctors determine the source of their seizures so surgeons could later remove the damaged regions.

First the patients had surgery to implant the electrodes to find out where the trouble was. After they had healed and the source was discovered, they had a second operation to eliminate the problem regions. Because the second surgery required putting the patients under anesthesia (in these cases with propofol), the already implanted electrodes also gave Purdon and his team a chance to “watch” the brain as it loses consciousness.

As the patients received the anesthesia, they listened to spoken words and verbally responded to indicate whether they heard their own name or a word for an object like chair or table. When they stopped responding, the researchers knew they had lost consciousness. Throughout the process, the implanted electrodes recorded brain activity both within and on the surface of the brain.

Those readings, says Purdon, could help neuroscientists determine the brain differences between consciousness and unconsciousness. As the patients went under, their self-awareness dissolved as various brain regions disconnected from one another.

“This study is providing us with new information about what’s really going on in propofol anesthesia,” says Dr. Nicholas Schiff, professor of neurology and neuroscience at Weill Cornell Medical College, who was not associated with the research. “I think it’s very important.”

As the different brain regions go off-line, a slow wave of electrical activity, which operates at a much-slower frequency than during wakefulness, starts to oscillate. “It’s slow, and it develops right at the point where the person loses consciousness,” says Purdon. “It’s much larger, an order of magnitude larger, than if you look at the same rhythm when someone is awake.”

These wave patterns restrict neuron activity so that cells can only fire for short periods of time. “[The new wave pattern] silenc[es the neurons] and only allow[s] them to be activated for brief periodic intervals,” Purdon says, “It’s not so much that the brain is per se, ‘shut off.’ It’s in some pattern that is incompatible with consciousness.”

The incompatibility essentially prevents different parts of the brain from communicating with each other, much like people speaking different languages have trouble conversing with one another. Although specific regions can show short periods of coherent firing patterns similar to those that govern normal waking states, the new oscillation pattern keeps them out of synch with each other so they can’t connect or share information.

“It’s as if a conductor is playing a piece where he shuts everyone in one section down and has another group play,” says Purdon.

The study adds support to the idea that consciousness is not found in any single brain region, but is rather the combined result of a network of neural connections between multiple areas of the brain. And while that has brain scientists eager to learn more about the biological basis of consciousness, the findings also have potential applications for patients. Very rarely — in about 1 in 10,000 operations — people can be awake while anesthetized. Using the signals found in the current study from the anesthetized patients, researchers may be able to develop an algorithm that provides a signature for unconsciousness. Anesthesiologists can then look for this pattern while their patients are going under, and ensure that they are truly asleep during surgery.

The research was published in the Proceedings of the National Academy of Sciences.